Field of the Invention
This invention relates generally to a system and method for assembling and compressing a fuel cell stack and, more particularly, to a system and method for assembling a fuel cell stack within an enclosure and compressing the fuel cell stack that is within the enclosure and then securing a cover over the stack and onto the enclosure such that stack compression is maintained.
Discussion of the Related Art
Fuel cells may be used to convert a fuel, such as hydrogen, into usable electricity through the use of an electrochemical reaction. Unlike internal combustion engines, fuel cells convert fuel into usable electricity without relying on combustion as an intermediate step. Thus, the use of fuel cells has environmental advantages when compared to internal combustion engines. A hydrogen fuel cell, for example, is an electro-chemical device that includes an anode and a cathode with an electrolyte therebetween. The anode receives hydrogen gas and the cathode receives oxygen or air. The hydrogen gas is dissociated in the anode to generate free protons and electrons. The protons pass through the electrolyte to the cathode, and react with oxygen and electrons in the cathode to generate water. The electrons from the anode cannot pass through the electrolyte, and thus are directed through a load to perform work in the form of direct current (DC) to the cathode via an external circuit that typically includes a load, such as an electric motor.
Proton exchange membrane fuel cells (PEMFCs) are a common type of fuel cell that is used for vehicle applications. The PEMFC generally includes a solid polymer electrolyte proton conducting membrane, such as a perfluorosulfonic acid membrane. The anode and cathode typically include finely divided catalytic particles, usually platinum (Pt), supported on carbon particles and mixed with an ionomer. The catalytic mixture is deposited on opposing sides of the membrane. The combination of the anode catalytic mixture, the cathode catalytic mixture and the membrane define a membrane electrode assembly (MEA). The delivery of reactants to the MEA, the removal of byproduct water, and the delivery of the generated electrical current to the load is facilitated via a gas-permeable layer (often referred to as a gas diffusion layer) and a bipolar plate. An anode gas diffusion layer is arranged in facing contact with the anode catalyst layer, while a cathode gas diffusion layer is arranged in facing contact with the cathode catalyst layer. As described herein, the MEA is understood to include the anode gas diffusion layer and the cathode gas diffusion layer. Each MEA is sandwiched between bipolar plates to create a fuel cell. During assembly, one MEA may be affixed to one bipolar plate by securing a planar surface of one of the electrodes (and one of the gas diffusion layers) to a planar surface of the bipolar plate to create a fuel cell that is ready for stacking, as is understood by those skilled in the art.
Several fuel cells are typically combined in a fuel cell stack to generate the desired power. For example, a fuel cell stack for a vehicle may have two hundred or more stacked fuel cells that are connected in series along a common stacking dimension, e.g., are stacked in a manner that is similar to a ream of paper, to form a fuel cell stack. As stated above, the bipolar plates are positioned on both sides of the MEAs of each fuel cell in a fuel cell stack. The stacked fuel cells are positioned between an end plate assembly and an end plate on each end of the stack. Each bipolar plate includes an anode side and a cathode side for adjacent fuel cells in the stack, where anode gas flow channels are provided on the anode side of the bipolar plates that allow for the anode reactant gas to flow to the respective MEA, and cathode gas flow channels are provided on the cathode side of the bipolar plates that allow the cathode reactant gas to flow to the respective MEA. One of the end plates of the fuel cell stack includes anode gas flow channels, and the other end plate includes cathode gas flow channels. Each of the end plate assemblies include an insulator plate and a current collector plate. The bipolar plates and the current collector plates are made of a conductive material such that the current collector plates conduct the electricity generated by the fuel cells through the bipolar plates and out of the stack. The insulator plates insulate the stack, as is known to those skilled in the art.
Integrating fuel cell stacks into automotive platforms requires precise placement and alignment with balance of plant equipment inside the vehicle's fuel cell system compartment, such as, by way of example, blowers, pumps, hoses, compressors, etc. The precise placement and alignment translates to tight dimensional tolerances of the assembled fuel cell stack. Additionally, fuel cell stacks that are in vehicles must be able to withstand stresses that are caused by acceleration, deceleration, crashes, accidents and various other impacts, and must also retain the position of the fuel cells in the stack relative to each other throughout the life of the fuel cell stack. High shearing force during the stresses described above may cause sliding between the cells of the stack, and small displacements between cells may result in large cell block displacements when all of the fuel cells are considered. Further, the problem of displacements between cells may be intensified by cold start conditions because thermally induced contraction may reduce a y-axis compression retention load that was placed on the cells during stack assembly.
The bipolar plates are sized such that they fit over and around the periphery of the MEAs that they are stacked in between. To improve alignment of each plate and MEA during assembly of the fuel cell stack, one or more datum structures, or simply datum, may be formed in part of or secured onto one or more of the bipolar plates. The datum may further include a bore that is configured to accept a datum pin to promote alignment of the bipolar plates and cells during the stacking process in a manner that is understood by those skilled in the art.
Fuel cell stacks are typically assembled under a compressive load so as to seal the fuel cells and to achieve low interfacial electrical resistance between the bipolar plates, the gas diffusion layers and the catalyst electrodes of fuel cells that make up the stack. The compressive load may be applied by a pressing agent, for example, a press. The compressive load on the fuel cell stack, which usually ranges from about 80 to 160 psi, depending on humidification, is maintained by a compression retention enclosure, also known as a housing, that encases the fuel cell stack. For example, to establish the desired compressive force, a press may be used to apply a load to the stack. Next, a compression retention system that includes a compression retention enclosure that houses the stack is affixed to the stack in a manner that maintains the compressive force on the stack after the press is released. Interconnecting tie rods or bracketing elements that are mounted along the surface of one or more of the side panels are typically included to bind the discrete components of the housing to maintain the compressive force on the fuel cell stack, or the housing may be a unitary structure that is affixed to the compressed fuel cell stack. Compressive force is retained by securing the tie rods with bolts or related fasteners such that the bolts are loaded in shear. Thereafter, the enclosure that houses the compressed fuel cell stack may be placed into another enclosure for environmental sealing or may simply use enclosing panels and seals for environmental sealing. Once completely housed and sealed, the fuel cell stack is mechanically secured, coupled to balance of plant (BOP) components, and electrically coupled to the vehicle or related device.
In another example, a press may be used to apply a load to a stack or a block of individual cells, and thereafter a housing can be lowered onto and over the stack. However, there is a need in the art to assemble a fuel cell stack in a housing that requires less time using a press to compress a stack and encase the stack in a compression retention system and that does not require a housing structure to be lowered onto and around the fuel cell stack that is being compressed.